The black liquor pollution of alkaline pulping and its treatment has been a world wide environmental problem. At present, the common way to treat the black liquor is to recover chemicals by combustion method. The detailed description of the mentioned method can be found in Pulp and Paper Manufacture (edited by E. W. Earl Malcolm, T. M. Tom Grace [American], translated by Cao Bangwei [Chinese], Chinese Light Industry Press, Beijing, 1998.6), wherein the technical process of said method summarily comprises: obtaining a green liquor by dissolving the smelt formed by evaporating and burning the black liquor, and obtaining an alkali liquor that is commonly named white liquor by causticizing the green liquor with excess lime. This method eliminates the pollution of black liquor and recovers alkali therefrom, but it generates a larger quantity of alkaline waste residue (named as lime mud), which consequently causes a serious secondary pollution. Currently, wood pulping mills generally calcine the lime mud to produce lime that can be recycled and reused in the process. However, this method needs a relatively high investment and energy consumption (about 250 kg of fuel oil is needed to produce one tone of lime) and brings about little economic value. In fact, this method is adopted only for eliminating the secondary pollution of lime mud. For straw pulping mills, the lime mud is not suitable for calcinations due to the high content of silicon in it, thus the lime mud is either introduced into rivers or lakes, or accumulated in dumping sites. For many years, numerous researchers in the world have been trying to develop a simple and feasible method for recovering or treating the lime mud, especially the lime mud generated in straw pulping mills. Unfortunately, the most of the researchers only focus on the treatment and utilization of the lime mud generated in the traditional chemical recovery process, and few of them have a way to avoid the formation of this waste residue (lime mud) from the beginning.
AI Tian-Zhao's patent application (CN1239166) disclosed “a new technology for causticizing green liquor generated in the process for recovering chemicals from black liquor in alkaline pulping”, which avoided the generation of lime mud and produced normal precipitated calcium carbonate while alkali was recovered. However, due to the higher reaction temperature and other limitations of process conditions, the calcium carbonate particles produced by said patent application were relatively large (with an average particle size around 5 μm), and could be used only in some situations of paper-making industry where calcium carbonate with a relatively large size is needed, and were not satisfactory for making coated paper or neutral sized paper or for other industries that need smaller particle sizes.
With the economical development, the need for the smaller size calcium carbonate, particularly those with an average particle size of less than 1 μm, has increased drastically in papermaking industry and other industries. Thus, studying a way to produce microfine or superfine precipitated calcium carbonate has become a hot research subject in the calcium carbonate industry in world. However, the research thinking is always focused on carbonization process, and there is no one who ever has thought on how to produce superfine precipitated calcium carbonate through the causticization reaction in the chemical recovery process of black liquor in alkaline pulping.